Various types of turbulator inserts are known for improving heat transfer and other flow modifying purposes, for example wound and twisted wire inserts, or metal strip inserts shaped and deformed to complex bent and twisted shapes to cause changes in direction of the fluid flow within the tube. This type of turbulator acts largely by sideways buffeting of the fluid flow which causes high pressure resistance and increases the energy needed to maintain the through flow. Moreover they are effective only within the localised length of the duct in which they are installed and have little or no effect downstream thereof.
Another form of known turbulator or flow modifying insert consists of a flat strip of metal equal in width to the tube internal diameter and twisted to form a helix so that the through flow follows a spiral path improving contact with the tube wall and also providing an enhanced effect downstream of the insert in that the gases exiting therefrom will continue to spin giving some additional scrubbing effect and wall contact after their exit from the spiral. This type of insert gives relatively low pressure resistance, however the spirally shaped strip is difficult and expensive to manufacture, particularly where costly high grade heat resisting metals are needed for high temperature and/or corrosive or other aggressive gases or other fluids.